HORTSCIENCE 55(4):503–506. 2020. https://doi.org/10.21273/HORTSCI14693-19 flowers form inflorescences that emit a strong, unique fragrance (Knuth, 1908; Kurz, 1997; McMinn, 1989; Nokes, 2001). Almost Cold Stratification and Pericarp entirely dioecious, P. trifoliata blooms in late May to June, and P. crenulata blooms in Removal Improve Seed Germination of April (Ambrose et al., 1985; Dirr, 1998). Ptelea trifoliata comprises five subspe- Ptelea trifoliata and Ptelea crenulata cies that occur in much of North America from Ontario to Florida and as far west as Anna J. Talcott and William R. Graves Nebraska and Arizona (Bailey, 1962; Yang Department of Horticulture, Iowa State University, 106 Horticulture Hall, and Applequist, 2015). Ptelea crenulata is Ames, IA 50011 found in California and P. aptera is found in Baja California (Yang and Applequist, 2015). Additional index words. dormancy, hoptree, polyethylene glycol, propagation, tetrazolium In Canada, 102 species of insects from 40 chloride, wafer ash families visit P. trifoliata and are thought to be pollinators (Ambrose et al., 1985). Leaves Abstract. Two species of North American shrubs in the genus Ptelea (Rutaceae), are eaten by larvae of swallowtail butterflies commonly known as eastern and western hoptree or wafer ash, have unfulfilled potential and grasshoppers (Scriber and Dowell, 1991; to increase the diversity of managed landscapes and support populations of pollinators and swallowtail butterflies. The white flowers of Ptelea are highly fragrant, and pistillate Sword and Dopman, 1999). The larvae of the flowers give rise to clusters of distinctive samaras. The insufficiency of information about moths Agonopterix pteleae Barnes and Busck improving germination of seeds of Ptelea trifoliata and lack of recommendations for and Agonopterix costimacula Clarke feed Ptelea crenulata prompted us to investigate effects of pericarp removal and cold (4 8C) exclusively on P. trifoliata in Illinois (Harrison stratification periods of 0, 4, 8, and 16 weeks on the germination of seeds of these two and Berenbaum, 2005). species. Samaras were collected from multiple plants of both species in the midwestern Several reports of the propagation of P. United States (P. trifoliata) and California (P. crenulata). The germination percentage of trifoliata from seeds are available (Dreesen viable seeds, calculated after tests of viability with tetrazolium chloride of seeds that did and Harrington, 1997; McLeod and Murphy, not germinate, increased with longer stratification periods, with 100% germination for P. 1977; USDA Forest Service, 1948). With 3 to crenulata and 91% germination for P. trifoliata after 16 weeks of stratification. The 4 months of cold stratification, 28% germi- germination value, a measure of the speed and uniformity of germination, and peak value nation was reported (USDA Forest Service, also increased with longer stratification in both species. Pericarp removal increased the 1948). Other researchers who stratified seeds germination percentage of both species and increased the peak and germination value of for 18 weeks found differences in germina- P. crenulata. Propagators seeking to grow these species of Ptelea from seed should remove tion based on samara size and color (Dreesen the pericarp and cold-stratify seeds for 16 weeks to improve germination success. and Harrington, 1997). The highest germina- tion (50%) occurred among the large, green- colored samaras that had the proximal end Ptelea trifoliata L. (Rutaceae) has been indehiscent, and light brown to cream in removed (Dreesen and Harrington, 1997). used as an ornamental shrub since the 18th color (Dirr, 1998). In the center of the fibrous, Higher germination (68%) was reported century and has gained international favor in papery pericarp are two cells that may con- when seeds were excised from pericarp and England and Scotland; however, it remains a tain two dark-colored seeds (USDA Forest stratified for 181 d (McLeod and Murphy, rare choice among landscapers (Harvey, Service, 1948). The small greenish-white 1977). Samara leachate inhibited germina- 1981; Lancaster, 1995). Two cultivars exist, pistillate and the creamy-white staminate tion, implying a chemical inhibitor (McLeod ‘Glauca’, which has dark green leaves, and ‘Aurea’, which has golden leaves (Dirr, 1998). Horticultural attention has focused on P. trifoliata; however, the genus com- prises two other species, Ptelea crenulata Greene and Ptelea aptera Parry (Yang and Applequist, 2015). Uncommonly used plants have the potential to add species diversity to landscapes, which can stabilize ecosystems (Tilman et al., 2006). Ptelea trifoliata and P. crenulata are un- derstory, shade-tolerant shrubs measuring 2 to 5 m in height with trifoliate leaves (Dirr, 1998; McMinn, 1989). The fruits are com- pressed samaras that are broadly winged, Received for publication 1 Nov. 2019. Accepted for publication 9 Jan. 2020. Published online 17 March 2020. Fig. 1. Stratification period (0, 4, 8, and 12 weeks) affects germination (%), germination distribution (d), We thank Frank Balestri for help with seed collec- peak value, and germination value of Ptelea crenulata and Ptelea trifoliata during Expt. 1. Peak value tion, Michael Stahr and the Iowa State University is defined as the cumulative germination percentage on the day of greatest germination divided by the Seed Laboratory for sharing equipment, and Chris- number of days to reach that level, germination distribution is defined as the span of days between the topher and Laura Yiesla for generous laboratory first and last germinating seeds, and germination value is defined as peak value multiplied by the mean assistance. daily germination, which expresses completeness and rate of germination. Mean daily germination is A.J.T. is the corresponding author. E-mail: atalcott@ defined as germination percentage per unit divided by number of days for germination. Species data iastate.edu. were pooled and designated with black dots for the germination percentage and germination This is an open access article distributed under the distribution because species were not significantly different from each other. A significant difference CC BY-NC-ND license (https://creativecommons. between P. crenulata (black circles and solid line) and P. trifoliata (open squares and dotted line) was org/licenses/by-nc-nd/4.0/). observed only for peak value and germination value. HORTSCIENCE VOL. 55(4) APRIL 2020 503 and Murphy, 1977). Despite the previous attention to seed germination in P. trifoliata, the need for a more precise procedure exists, and there is no information about the germi- nation of seeds of P. crenulata. Soaking seeds in water with dissolved polyethylene glycol (PEG), called osmopriming, has been used to increase the germination rates of various crops (Farooq et al., 2005; Naglreiter et al., 2005; Salehzade et al., 2009). PEG decreases the water potential difference be- tween dry seeds and surrounding moisture. PEG is physiologically inert because its large molecular mass prevents it from entering cell membranes (Naglreiter et al., 2005). Rapid imbibition can lead to reduced cellular respi- ration and germination (Powell and Matthews, 1978); in theory, PEG should facilitate more gradual imbibition. Our objective was to optimize the speed, uniformity, and completeness of germination of P. trifoliata and P. crenulata with strati- fication and pericarp removal. During our first experiment, the effects of stratification duration were compared with seeds that had been separated from their pericarps. During our second experiment, the effects of two methods of pericarp removal were compared with use of intact, whole samaras because it is not clear how McLeod and Murphy (1977) removed pericarps. Both our removal methods involved soaking samaras to soften the peri- carp, but one was soaked in a PEG solution to determine if rapid imbibition had any effect on seed vitality. Materials and Methods Expt. 1: Stratification effects. Mature, dried samaras from four plants from a small population of P. crenulata from Black Dia- mond Mines Regional Preserve in Antioch, CA were collected with permission on 11 Sept. 2016. Mature, dried samaras from two cultivated plants of P. trifoliata were col- lected on 21 Sept. 2016 from the Morton Arboretum in Lisle, IL, where five hoptrees are present. These source plants had been grown from seeds collected in the wild along the Apple River in Jo Daviess County, IL, and Essex County, Ontario, Canada. After being softened by soaking in tap water for 12 to 24 h, samara pericarps were torn in two by hand to remove the seeds. Seeds were strat- ified at 4 °C for 0, 4, 8, and 16 weeks in damp sphagnum moss in petri dishes placed inside sealed plastic bags to retain moisture. The experimental unit of 20 seeds was replicated Fig. 2. During Expt. 2, the prestratification treatment groups were as follows: whole unsoaked samaras six times for each stratification period. For (control); seeds removed from softened pericarp after soaking overnight in water (water); and seeds P. trifoliata, three replicates from each treat- removed from softened pericarp after soaking in polyethylene glycol 8000 solution (–0.6 MPa; ment contained seeds from two plant sources. PEG). Bars represent means of germination (%), peak value, and germination value of Ptelea For P. crenulata, all six replicates contained crenulata (black) and Ptelea trifoliata (gray). Peak value is defined as the cumulative germination seeds from all four plant sources. percentage on the day of